I may be wrong here, but isn't it only the Royal Family of Dol Amroth that has the Elven Blood? And I think it mentions something about making these types of characters in the LotRs Corebook, not sure where though.
If Prince Imrahil does not qualify for the Core Book's "elven blooded" sidebar, I don't know who would. I would just use that for the Princes and other high family members. I would not give the "common man" any added benefits.